Low-Density Microcellular Carbon Materials
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SEM shows that the average cell/pore size for thèse materials is much less than one micrometer. In fact, noninvasive techniques such as SAXS reveal cell/pore radii of 50-150 Â, depending on density and synthetic conditions. The effect of the [Resorcinol]/[Catalyst] ratio on the density of RF aerogels also manifests itself during pyrolysis. In ail cases, a 50% mass loss is experienced u p o n p y r o l y s i s to 1050°C, b u t the amount of volumetric shrinkage and the accompanying densification are linked to the R/C ratio in the original formulation. For example, at 5% solids RF aerogels prepared under high catalyst conditions (R/C = 50) expérience a —100% density increase in going from the crosslinked polymer to pure carbon, but only a —40% increase under low catalyst conditions (R/C = 300).
100À
Figure 5.1 SEM (a) and TEM (b) micrographs ofa carbonized aerogel synthesized at R/C = 200 and pyrolyzed at 1050°C. Aerogel density equals 0.1 g/cm'.
transparent. Nevertheless, the aerogel microstructure is retained. Figure 5.1 shows the microstructure of a carbon aerogel as revealed by SEM and TEM. The TEM s h o w s that the aerogel is composed of interconnected colloidal-like particles giving a "stringof-pearls" morphology analogous to that of the starting RF aerogel. Of course, the size and shape of the particles are slightly altered after pyrolysis, but the same gênerai microstructure is retained.
Interestingly, the particle size for the high catalyst aerogel is slightly larger in the carbonized state when compared to the initial RF material. It appears likely that the small, individual particles of the RF aerogel fuse together during pyrolysis, leading to slightly larger particles in the carbonized state. Such "healing" phenomena hâve been observed in the pyrolysis of other polymers.'- 3 Under low catalyst conditions, the particle size in the carbonized state is slightly smaller than in the corresponding RF aerogel. The change in particle size after carbonization is reflected in the surface area (BET method) data shown in Figure 5.2. The microstructural différences of carbon aerogels are apparent in their mechanical properties. 4 - 5 Figure 5.3 shows the compressive modulus of carbon aerogels as a function of density and R/C ratio. The linear log-log plot in each case demonstrates a power-law density dependence. The scaling exponents (2.7±0.2) for the carbon aerogels are identical to their RF precursors. In addition, the same rank order is observed for the various formulations; that is, carbon aerogels prepared under high catalyst c o n d i t i o n s (R/C = 50) h â v e higher moduli than their low catalyst (R/C = 300) counterparts. In gênerai, carbon aerogels are about 10 times stiffer than their RF analogs at équivalent densities and R/C ratios. To ascertain the effects of pyrolysis on the aerogel microstructure, it is helpful
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Low-Density Microcellular Carbon Materials
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Figure 5.2 ComparisonofBET surface a
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